Image sensor with dark reference pixel oversampling

Abstract

An image sensor may include an array of image pixels arranged in rows and columns. A first portion of the array may include active pixels that are read out using first analog-to-digital converter (ADC) circuits. A second portion of the array may include dark reference pixels that are read out using second analog-to-digital converter (ADC) circuits. The first ADC circuits may have a first sampling rate, a first resolution, and a first size. The second ADC circuits may have an oversampling rate that is greater than the first sampling rate, a second resolution that is greater than the first resolution, and a second size that is bigger than the first size. Configured in this way, the second ADC circuits may perform averaging of a row noise via direct time-domain oversampling, which can help dramatically reduce the number of dark reference pixel columns in the array.

What is claimed is: 
     
         1 . An image sensor, comprising:
 active pixels configured to receive light;   dark reference pixels;   first analog-to-digital converter (ADC) circuits configured to receive signals from the active pixels and configured to operate at a first sampling rate; and   second analog-to-digital converter (ADC) circuits configured to receive signals from the dark reference pixels and configured to operate at a second sampling rate greater than the first sampling rate.   
     
     
         2 . The image sensor of  claim 1 , wherein the dark reference pixels comprise optically black pixels. 
     
     
         3 . The image sensor of  claim 1 , wherein the dark reference pixels comprise electrically black pixels. 
     
     
         4 . The image sensor of  claim 1 , wherein the second ADC circuits comprise oversampling analog-to-digital converters. 
     
     
         5 . The image sensor of  claim 4 , wherein the second ADC circuits comprise sigma-delta analog-to-digital converters. 
     
     
         6 . The image sensor of  claim 4 , wherein the first and second ADC circuits are different types of analog-to-digital converters. 
     
     
         7 . The image sensor of  claim 1 , wherein the first ADC circuits exhibit a first resolution, and wherein the second ADC circuits exhibit a second resolution that is greater than the first resolution. 
     
     
         8 . The image sensor of  claim 1 , wherein the first ADC circuits exhibit a first size, and wherein the second ADC circuits exhibit a second size that is larger than the first size. 
     
     
         9 . The image sensor of  claim 1 , wherein the second ADC circuits are used to read signals out from the dark reference pixels to obtain an estimated row noise correction value. 
     
     
         10 . The image sensor of  claim 1 , wherein sample-and-hold circuits are formed at inputs of the first ADC circuits, and wherein no sample-and-hold circuits are formed at inputs of the second ADC circuits. 
     
     
         11 . The image sensor of  claim 1 , wherein the dark reference pixels form less than 100 dark reference pixel columns in the image sensor. 
     
     
         12 . The image sensor of  claim 1 , wherein the dark reference pixels form less than 50 dark reference pixel columns in the image sensor. 
     
     
         13 . The image sensor of  claim 1 , wherein the dark reference pixels form less than 10 dark reference pixel columns in the image sensor. 
     
     
         14 . The image sensor of  claim 1 , wherein the dark reference pixels form only one dark reference pixel column in the image sensor. 
     
     
         15 . A method of operating an image sensor, comprising:
 using active pixels to receive light;   using dark reference pixels to estimate noise;   using first analog-to-digital converters (ADCs) to receive signals from the active pixels, wherein the first ADCs have a sampling rate; and   using second analog-to-digital converters (ADCs) to receive signals from the dark reference pixels, wherein the second ADCs have an oversampling rate that is greater than the sampling rate of the first ADCs.   
     
     
         16 . The method of  claim 15 , wherein using the second ADCs to receive signals from the dark reference pixels with the oversampling rate comprises averaging out the noise in the time domain to reduce the number of dark reference pixels in the image sensor. 
     
     
         17 . The method of  claim 15 , wherein the first ADCs receive signals from the active pixels while the second ADCs receive signals from the dark reference pixels. 
     
     
         18 . The method of  claim 15 , further comprising:
 using the first ADCs to compute active pixel readout values;   using the second ADCS to estimate the noise; and   computing noise corrected pixel values by subtracting the estimated noise from the active pixel readout values.   
     
     
         19 . Imaging circuitry, comprising:
 a pixel configured to receive light;   a dark pixel;   a first analog-to-digital converter (ADC) configured to receive signals from the pixel, wherein the first ADC has a first resolution; and   a second analog-to-digital converter (ADC) configured to receive signals from the dark pixel, wherein the second ADC has a second resolution that is different than the first resolution.   
     
     
         20 . The imaging circuitry of  claim 19 , wherein the second resolution of the second ADC is greater than the first resolution of the first ADC. 
     
     
         21 . The imaging circuitry of  claim 19 , wherein the second ADC is an oversampling analog-to-digital converter.